1. Field of the Invention
The present invention relates to conductive polymers, and more particularly to conductive polymers which have highly enhanced solubility in organic solvents and electrical conductivity, and synthesizing process thereof.
2. Discussion of the Related Art
Conductive polymers have conjugated structures along double bonds present in the backbone thereof and has much enhanced electrical conductive properties compared to other organic materials because the conductive polymers form partial electrical charges along the conjugate structures and thereby having unlocalized electrons when the polymers are doped with dopants such as a protonic acid. Because the conductive polymers have both enhanced electrical, magnetic or optical properties comparable with conventional metals and satisfactory mechanical properties and processability as conventional polymers, they have been remarkably attracted in the filed of chemistry, physics, material engineering and industries.
The first developed conductive polymer is polyacetylene, which was developed by Shirakawa et al., however, polyacetylene is oxidized easily in the air. After polyacetylene was developed, Conductive polymers such as polyaniline, polypyrrole, and polythiophene have been developed.
The conductive polymers can be used in various applications according to their electrical conductivity. For example, the conductive polymers with electrical conductivity of 10−13˜10−7 (S/cm), 10−6˜10−2 (S/cm), and equal to and more than 100 (S/cm), respectively have been used as antistatic materials, static discharge materials, and electro-magnetic interference(EMI) shielding materials, battery electrodes, semiconductor and solar cells. Accordingly, the conductive polymers may be utilized in more various applications by improving their electrical conductivities.
Among intrinsically conducing polymers, polyaniline has been noticed in the relevant field since it is not only cheap and very stable compared to polypyrrole and polythiophene also doped easily by protonic acids.
The polyaniline (PANI) can be classified into the completely reduced from, leucoemeraldine, the intermediated oxidized form, emeraldine, and the fully oxidized form, pernigraniline, according to its oxidation state.
However, the conductive polymers synthesized through the conventional processes, especially the polyaniline as the completely reduced from, leucoemeraldine, the intermediated oxidized from, emeraldine salt, and the fully oxidized form, pernigraniline, have disadvantages that they cannot be made from melting process owing to their high boiling point and that they must experience complex processing steps since they have low solubility in solvents with high-boiling point or universal or compatible solvents such as meta-cresol.
In order to improve the problems of the conductive polymers as indicated above, copolymers such as aniline derivatives or graft copolymers have been synthesized by inducing various side chains into the benzene ring or amine group of the conductive polymers for improving solubility of the backbone of the conductive polymers. Alternatively, various dopants, or other organic materials, polymers or plasticizers are added into the conductive polymer for improving the processability and the electrical conductivity of the conductive polymers. However, those composites have lower electrical conductivity compared to the conductive polymers before reforming.
Polyaniline (PANI) can be synthesized either by electro-chemical charge transfer reaction which uses electro-chemical reaction or by chemical oxidation process that uses protonation through acid-base or redox reaction. However, it has been known that the chemical oxidation process is suitable for producing polyaniline in industrial scales.
Representative chemical oxidation process for synthesizing polyaniline has been reported to MacDiarmid et al., who synthesized polyaniline by polymerizing aniline monomers dissolved in hydrochloric acid with oxidizing agents such as ammonium persulfate in aqueous solution in the temperature of 1˜5° C., separating and washing the precipitates and then obtaining polyaniline (See A. G. MacDiarmid, J. C. Chiang, A. F. Richter, N. L. D. Somarisi, in L. Alcacer (ed.), Conducting Polymers, Special Applications, Reidel, Dordercht, 1987, p. 105). The MadDiarmid process have been utilized widely and regarded as a standard method for producing polyaniline.
The polyaniline of emeraldine base (EB) synthesized according to the MacDiarmid process has low molecular weight (intrinsic viscosity 0.8˜1.2 dl/g), but it is dissolved in 1-methyl-2-pyrrolidon (NMP). Also it has reported that emeraldine salt produced by doping the EB with 10-camphorsulfonic acid (ES-CSA) is dissolved a little in meta-cresol. The film made from that solution containing ES-CSA has at most electrical conductivity of about 100 S/cm, on the other hand, the film made from emeraldine salt doped with hydrochloric acid (ES-HCl) shows highly lower electrical conductivity of about 5 S/cm. However, it needs to be separating not dissolving portion from the dissolved portion in the MacDiarmid process. Especially, the polyaniline synthesized according to the MacDiarmid process has low molecular weight, broad molecular weight distribution, and inferior solubility to solvents or electrical conductivity resulted from side chain reactions to the backbone. Therefore, there remains a need of improving the micro-chemical structure or electrical conductivity of polyaniline synthesized according to the MacDiarmid process.
In order to improve the disadvantages and inferior processability of the polyaniline synthesized by MacDiarmid process, a lot of researches which use emulsion polymerization have been suggested. For example, U.S. Pat. No. 5,232,631 and No. 5,324,453 to Cao et al., which are incorporated herein by reference, disclose process for synthesizing polyaniline by dissolving aniline monomers and functionalized protonic acid in polar solvents such as water, mixing the solution with an organic solvent to prepare an emulsion, and then adding an oxidizing agent into the emulsion. Cao et al. reported that the emeraldine salt (ES) can be dissolved in nonpolar solvent such as xylene because emulsifier acts as a dopant, and therefore, it is reacted with the polyaniline to form composite. However, since Cao et al. uses functionalized protonic acids as emulsifier, it is difficult to control doping the emulsifier and the process requires commonly expensive material. Further, since the functionalized organic acid is hardly separated from polyaniline after polymerizing reaction, the conductive polymers may have only very limited uses and highly inferior electrical properties. For instance, the emeraldine salt, which synthesized according to Cao et al, doped with dodecyl benzene sulfonic acid (DBS) has a solubility of less than 0.5% and an electrical conductivity of only about 0.1 S/cm.
Kinlen of Monsanto produced polyaniline salt by preparing reverse emulsion system comprising an organic solvent such as 2-butoxyethanol soluble in water and an organic acid, which is not soluble in water but soluble in the organic solvent, as a hydrophobic emulsifier, mixing an aniline monomer and a radical initiator with the emulsion system and polymerizing the mixture to form polymer solution that has an organic layer, which contains polyaniline salt, separated from an aqueous layer containing the radical initiator and non-reacting compounds. (See U.S. Pat. No. 5,567,356; Kinlen, Macromolecules, 31, 1745 (1998), which are incorporated herein by reference). Kinlen reported that the polyaniline salt was soluble in nonpolar solvents of no less than 1% (w/w). However, it is difficult to synthesize polyaniline because the radical initiator in the aqueous layer is separated from the monomer in the organic layer and polyaniline synthesized according to Kinlen process has low electrical conductivity owing to difficulty of control doping process. For example, it was reported that polyaniline salt synthesized with dinonyl naphthalene sulfonic acid as a hydrophilic organic acid had an electrical conductivity of about 10−5 S/cm in case the salt is manufactured as pellets.
Harlev et al. synthesized polyaniline salt with the MacDiarmid process except using pyruvic acid instead of hydrochloric acid (See U.S. Pat. No. 5,618,469, which is incorporated herein by reference). It is possible to improve processability of polyaniline by using pyruvic acid because pyruvic acid functions as organic solvent as well as dopant. However, since pyruvic acid has lower acidity it is difficult to dope polyaniline by pyruvic acid. Accordingly, polyaniline doped with pyruvic acid has low electrical conductivity, and especially in case the polyaniline doped with pyruvic acid is used as a transparent electrode, it has very high apparent surface resistance as much as 20,000 Ω/square, which is very high electrical resistance for the transparent electrode.
Ho et al., produced polyaniline through emulsion system prepared by adding specific emulsifier into an organic mixture solvent comprising an aniline monomer and a protonic acid with stirring (See U.S. Pat. No. 6,030,551, which is incorporated herein by reference). According to Ho et al., both a radical initiator such as benzoyl peroxide and the polyaniline is dissolved in the same non-aqueous layer, and therefore, it is possible to synthesize polyaniline solution in situ without residual solids. However, since it is not easy to separate the non-aqueous layer from an aqueous layer, it is expected that polyaniline synthesized according to Ho process may not have high electrical conductivity.
U.S. Pat. No. 6,072,027 to Carey et al., which is incorporated herein by reference, discloses a producing method of polyaniline with highly enhanced polymerization yield, by using chlorate salt or hydrochloric acid combined with bi- or trivalent iron salt as a new oxidization initiator.
Palaniappan et al. disclose a process for the preparation of polyaniline salt by forming inverted emulsion system that comprises an aqueous layer and an organic layer using a surfactant and then polymerizing the inverted emulsion system at room temperature using a radical initiator such as benzoyl peroxide dissolved in the organic layer (See U.S. Patent publication No. 2002-00062005, U.S. Pat. Nos. 6,586,565 and 6,630,567, which are incorporated herein by reference). However, polyaniline film prepared from Palaniappan process has very low electrical conductivity, for example about 0.1 S/cm, and may be only used in much limited applications since it is impossible to raise molecular weight of polyaniline.
In addition to emulsion polymerization as above, polyaniline synthesis processes through a dispersion polymerization, in which monomers such as aniline is fully dissolved in reacting solvent while synthesized polymers are not dissolved in the solvent, have been reported. For example, Armes et al. reported the polymerization process which comprises stabilizing sterically the conductive polymer by designing particular stabilizer and then particularizing the conductive polymer (See Armes et al., handbook of Conducting Polymers, Elsenbaumer ed. M. Dekker, New York, 1996, Vol. 1, p. 423). In this dispersion polymerization, since most of the stabilizer covers with the polyaniline, the polyaniline in aqueous solution can be prepared. However, the synthesized polyaniline has a particle size of about 60˜300 nm, which is affected by the stabilizer, and has low electrical conductivity, which defines its application.
Further, there have been reported that polyaniline is synthesized in aqueous solution containing organic solvents. Geng et al. prepared polyaniline film, which has electrical conductivity of about 10 S/cm, through synthesizing polyaniline with organic solvents such as ethanol, THF, and acetone (See Geng et al., Synth. Metals. 96, 1 (1998)). However, since it needs very long polymerization reaction time in Geng process, a probability of side reaction is raised.
According to Beadel et al., the polyaniline produced by the standard synthesizing method disclosed in MacDiarmid as described above has higher electrical conductivity as it has higher molecular weight. Accordingly, the monomer needs to be reacted or polymerized at lower temperature in order to enhance molecular weight of the polymer (See Beadel et al., Synth. Met. 95, 29˜45, 1998). For lowering reacting temperature, when aniline monomer is polymerized in homogeneous aqueous solution system, metallic salts such as LiCl, CaF2 and the likes are usually added to the system in order to prevent the system from freezing. However, mixing those metallic salts with the solution system causes the reaction to being slow, that is to say, at least 48 hours to complete the polymerizing reaction, and therefore, it is difficult to control the polymerization reaction. Also, as lowering the reaction temperature, the synthesized polyaniline has an increased molecular weight as well as molecular weight distribution (polydispersity of equal to or more than 2.5).
Also, there form side chains as the aniline monomer is added into a quinonediimine group in intermediate chains. Accordingly, FeCl2 as an oxidizing agent is added during polymerization reaction in order to inhibit formations of the side chains in polyaniline, or the polyaniline is eluted with organic solvents for removing side products such as oligomers which quit synthesis during the polymerization reaction. Besides, since the monomers are added into the polyaniline on the ortho-positions as much as the para-positions of the benzene ring in the polyaniline backbone in case of emulsion polymerization or interfacial polymerization, such synthesized polyaniline has much side chains, which cause the polyaniline to have lower electrical conductivity and solubility.
According to Thyssen et al., there is a probability of about 10% of the ortho coupling, which induces side chains in the backbone of the polymers, when the aniline monomers are polymerized by using electro-chemical process (See Thyssen et al., Synth. Met. 29, E357˜E362, 1989). Such polymers synthesized by ortho-coupling has lower hydraulic dimensions, which results in decreased intrinsic viscosity, compared to polymers synthesized by para-coupling, i.e. polymers without side chains. In other words, the polymers synthesized by ortho-coupling has much side chains and has more molecular weights even though they have low intrinsic viscosity of equal to or less than 1.2 dl/g. Accordingly, the polymers synthesized by ortho-coupling has inferior processability without improving the electrical conductivity.
Moreover, Huang et al. produced polyaniline of nano-fiber form by preparing a system which comprises an organic layer and an aqueous layer immiscible with the organic layer, dissolving an aniline monomer in the organic layer, and an initiator and an organic acid in the aqueous layer, and polymerizing the monomer in the interface (See Huang et al., J. Am. Soc. 125, 314 (2003)).
Min of Dupont Technology reported that conductive polymer with high yield could be obtained by increasing level of LiCl or NaCl as additive in the MadDiarmid process, as described above, up to 5-10 M at 0° C. for 3 hours (See G. Min, Synth. Met., 119, 273, (2001)).
In addition to the patents and references described above, many researches were reported for improving physical or chemical properties, for example electrical conductivity of the conductive polymers (Organic Conductive molecules and Polymers, Vol. I-IV, Ed. By H. S, Nalwa, John Wiley & Sons, New York, 1997; Handbook of Conducting Polymers Vol. I, II, Ed. By Skotheim et al., Marcel Dekker, New York, 1998; Conductive Polymers, P. Chandrssekhar, Kluwer Acade. Pub. Boston, 1999; Conductive Electroactive Polymers by G. G. Wallace, G. M. Spinks, L. A. P. Kane-Maquire, P. T. Teasdale, 2nd ed. CRC Press, New York, 2003).
However, the polyaniline producing processes disclosed to date make use of introducing substituents into monomers or mixing the monomers with immense amount of additives such as stabilizer or emulsifier, and therefore, it is difficult to obtain pure polyaniline. Also, because polyaniline according to conventional processes has synthesized by ortho-coupling as much as para-coupling and frequently forms side chains by side reactions, such polyaniline does not have high electrical conductivity, which limits its applications.
Also, polypyrrole has been synthesized mainly by electro-chemical synthetic process. In case of synthesizing polypyrrole by the electro-chemical process, unlike synthesizing polyaniline, acids are not added during polymerization, which makes the reaction simplify. However, when polypyrrole is synthesized according to the chemical process, side reactions such as inter-chain crosslink or side chain addition to the backbone of polypyrrole frequently happened, which causes synthesized polypyrrole to not dissolving in common solvents and therefore deteriorates processability. In case of synthesizing polypyrrole by electrical process, solvents or counter ions of conductive plate have highly affects on physical properties of polypyrrole.
Lee et al. prepared of conductive polypyrrole powder by reacting pyrrole monomers at 0° C. for 40 hours using chloroform and dodecyl benzene sulfonic acid (DBSA) of the same molar equivalents of chloroform (See J. Y. Lee, D. Y. Kim, C. Y. Kim, Synth. Met. 74, 103 (1995)). DBSA adopted by Lee et al. acts as both dopant and surfactant. However, polypyrrole film sample synthesized by Lee et al. has very low electrical conductivity of about 5 S/cm.
Besides, chemical process for synthesizing polypyrrole in organic solvents such as CHCl3, THF, or CH3N2O has been tried in order to produce polypyrrole being able to dissolve in the organic solvents. However, such synthesized polypyrrole did not have electrical conductivity at all.
Ames et al. reported a process for preparing stable colloidal polypyrrole by using poly-vinylalcohol, poly-ethyleneoxide, or poly-vinylpyridine as a steric stabilizer dissolved in water (See Armes et al., Handbook of Conducting Polymers Elsenbaumer ed. M. Dekker, New York, 1996, Vol. 1, p. 423). However, since polypyrrole powders are surrounded by a lot of stabilizers, like polyaniline, and therefore polypyrrole has a very low electrical conductivity.
Accordingly, it may enhance electrical conductivity of polypyrrole by linking pyrrole monomers on 2, 5 positions between pyrrole ring and sustaining linearity thereof. As mentioned above, pyrrole may be soluble in much solvents compared to aniline, however, it is very difficult to dissolve oxidizing agent and pyrrole monomer in the same solvent.
The synthetic conductive polymers, especially polyaniline has much lower real electrical conductivity than theoretically calculated electrical conductivity, about 105˜106 S/cm (Kohlamn et al., Phys. Rev. Lett. 78(20), 3915, 1997), because they does not have fully linear form and form completely orders such as crystalline structure per se. Since such polymers with lower electrical conductivity cannot be utilized as transparent plastic electrode or EMI shielding materials, there still remain needs of development of polyaniline having much improved electrical conductivity in the related field.